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Page 63
6.6 Post-D combiner.
Post D combiner is advised when using frequency diversity in
FM mode
.
Using the post D combiner, for proper combining, the maximum delay between the RF inputs
should not exceed 20% of bit duration (At 10 Mbit/s, this is 20 ns), taking into account:
The embedded delay on :
-
The PCM data streams.
-
The RF cables towards the TX antenna.
The embedded delay on :
-
The RF cables from the antenna to the receivers.
In case of space diversity :
-
The propagation delay between the two antennas.
6.6.1 Post-D
– best channel.
In this case, the two demodulated signal are only switched taking into account the AGC
information. This mode is used while transmitting data.
6.6.2 Post-D in optimum mode.
In this case, the two demodulated signal are weighted by the AGC information and summed. This
mode is used while transmitting an analog signal in FM (Typically a video signal). Due to
nonlinear behavior of FM modulation, when transmitting PCM, no gain is observed in optimum
mode and the best channel mode is recommended.
